Many digital communication designs include differential output buffers. Two common differential output buffers are Low-Voltage Differential Signaling (LVDS) and Differential Current Mode Logic (CML). A typical LVDS differential output buffer 100 is depicted in FIG. 1, while a typical CML differential output buffer 200 is depicted in FIG. 2.
The LVDS differential output buffer 100 is usually fabricated using four CMOS transistors 102-108 connected in a bridge formation as shown in FIG. 1. The differential output buffer 100 also includes a current source 110. The differential output buffer 100 operates by steering current through a termination resistor 112, which is typically 100 Ohms, located at a remote receiver 114. For a logic-1, current flows from the current source 110 through the transistor 104, through the termination resistor 112, and to ground through the transistor 106. For a logic-0, current flows from the current source 110 through the transistors 102, 108 and the termination resistor 112.
During steady-state conditions, the transistors 102-108 are either in saturation or cutoff, and current does not flow from the current source 110 to ground without going through the termination resistor 112. While this provides for low-power operation at low frequencies, it limits the speed at which the transistors are able to switch.
The CML differential output buffer 200 is typically fabricated using two resistors 202, 204 and two transistors 206, 208 connected in a bridge formation as shown in FIG. 2. The transistors 206, 208 may be CMOS transistors as depicted in FIG. 2, bipolar transistors, or any other suitable transistor type. The differential output buffer 200 also includes a current source 210. For a logic-1, current travels though one of the resistors 202, 204, through a termination resistor 212 (typically 100 Ohms, located at a remote receiver 114), through one of the transistors 206, 208, and into the current source 210. For a logic-0, the current flows through the other resistor and transistor in the output buffer 200.
The operation of the CML differential output buffer 200 is similar to the LVDS differential output buffer 100, except the transistors 206, 208 operate in the linear range. Because of this linear operation, there is a small current that flows from the current source to ground through both resistors 202, 204 and transistors 206, 208 in the CML differential output buffer 200. By operating the transistors 206, 208 in their linear range, the CML differential output buffer 200 can operate at very high frequencies.
A differential output buffer, such as the buffers depicted in FIGS. 1-2, may be used in applications in which the buffer is exposed to harsh environments, such as space and military applications. However, the current sources and transistors in the differential output buffers may be susceptible to Single Event Effects (SEE). SEE is a disturbance in an active semiconductor device caused by a single energetic particle. As semiconductor devices become smaller and smaller, transistor threshold voltages decrease. These lower thresholds reduce the charge per node needed to cause errors. As a result, the semiconductor devices become more and more susceptible to transient upsets.
One type of SEE is a single event upset (SEU). SEU is a radiation-induced error in a semiconductor device caused when charged particles lose energy by ionizing the medium through which they pass, leaving behind a wake of electron-hole pairs. The electron-hole pairs form a parasitic conduction path, which can cause a false transition on a node. The false transition, or glitch, can propagate through the semiconductor device and may ultimately result in the disturbance of a node containing state information, such as an output of a latch, register, or gate.
Typically, an SEU is caused by ionizing radiation components, such as neutrons, protons, and heavy ions. The ionizing radiation components are abundant in space and at commercial flight altitudes. Additionally, an SEU may be caused by alpha particles from the decay of trace concentrations of uranium and thorium present in some integrated circuit packaging. As another example, an SEU may be caused by detonating nuclear weapons. When a nuclear weapon is detonated, intense fluxes of gamma rays, x-rays, and other high energy particles are created, which may cause SEU.
Thus, it would be beneficial to harden a differential output buffer so that the buffer may be used in applications that are susceptible to SEE.